The advent in 1964 of program-controlled switching systems, such as the lESS manufactured by the Western Electric Co. and described in the September 1964 issue of the Bell System Technical Journal, has increased the demand for enhanced calling services including add-on, call forwarding and call waiting. These services are relatively easy to provide in stored program controlled switching systems because it is merely necessary to load a description of the service functions to be provided into the memory unit of the system. As the use of such stored program controlled switching systems has become more widespread the public has become accustomed to expect that such services will be available everywhere. This expectation extends as well to those customers who are still served by electromechanical offices of the local crossbar type. Unfortunately, it is not always economically attractive to provide enhanced services to customers served by crossbar equipment and it also may be impractical to immediately replace all existing crossbar offices with stored program controlled equipment.
The local crossbar switching system was introduced shortly after World War II and is disclosed in A. J. Busch U.S. Pat. No. 2,585,904 issued Feb. 19, 1952 as well as in a number of articles which have appeared in the Bell Laboratories Record, beginning in October 1949. For purposes of understanding the crossbar technology context of the present invention, the following Record articles are of interest: "No. 5 Crossbar Marker" by A. O. Adam, November 1950, page 502; "Number Group Frame for No. 5 Crossbar" by O. J. Morzenti, July 1950 page 298 as well as the article entitled "No. 5 Crossbar" by F. A. Korn and J. G. Ferguson which appeared in Electrical Engineering, August 1950, page 679.
In the prior art it had been proposed to provide enhanced calling services, such as call waiting, in crossbar systems by providing each line to be accorded this service with a line circuit having an auxiliary appearance on the crossbar line link frame. Examples are shown in M. E. Krom U.S. Pat. No. 3,320,367 issued May 16, 1967 and in A. Zarouni U.S. Pat. No. 3,133,995 issued May 19, 1964. When a line having the auxiliary line appearance feature is called and is found to be busy on a first call, the crossbar "marker" (see next paragraph) would connect the second call to the auxiliary line appearance of the called line and a distinctive tone signal would be heard by the called subscriber while regular ring-back tone would be provided to the calling party making the second call. The busy subscriber may then flash the telephone switchhook in response to the distinctive tone to hold the first call and take the second call. The busy subscriber may then return to the first call by flashing again, etc.
The local crossbar switching system marked a great departure from prior switching systems in that the directory number that could be assigned to a subscriber's line was independent of the physical location of that line in the crossbar equipment. The flexible assignment of directory numbers was achieved through the use of an electromechanically changeable translator known as a number group frame. This translator received the called directory number from the common control device called a "marker" and, in turn, furnished the marker with the physical location of the called line in terms of that line's line link frame number, vertical group, horizontal group and vertical file information. The marker then completed a connection between the calling and called line.
In retrospect, it may now be appreciated that the number group frame performed a translation function which may be roughly compared to the similar usage of the memory unit in a stored program controlled switching office. The electromechanical number group frame, however, is capable of performing only a very few of the myriad translation process that can be accomplished through the use of electronic memory storage and is just not "reprogrammable" to store sophisticated translations. For example, as described in H. S. Pat. Bean U.S. Pat. No. 3,278,692 issued Oct. 11, 1966, the number group may be modified so that it furnishes the marker not only with the equipment location of the called line but also with the units digit of the directory number for the called line's auxiliary line appearance. When the marker determines that the called line's primary equipment location is busy, the marker would re-cycle and seize the number group again, this time replacing the units digit of the originally called directory number with the units digit that it received from the number group on the first seizure. The second call would then be connected to the auxiliary line appearance.
While the number group allows flexible assignment of directory number to equipment locations, the above-described prior art crossbar systems nevertheless requires that each line equipment location have a corresponding (though not necessarily published) directory number. The requirement that every line appearance, including the auxiliary line appearance, must have a directory number is particularly disadvantageous in areas where the available pool of directory numbers is nearing exhaustion. Because call-waiting service is employed at the point of call termination rather than at the point of call origination, the effect of the required withdrawal of a directory number is felt at the terminating office. Where that office may be "short" of available directory numbers the telephone company may be reluctant to "throw away" a scarce directory number that could be assigned to usage-sensitive call origination and dedicated that number to enhanced call completion service for which only a small monthly fee is charged to the called subscriber desiring the service.
As further pointed out in E. D. Masucci U.S. Pat. No. 3,479,465 issued Nov. 18, 1969, the two-line hunting arrangement of the Bean patent is employed most efficiently only where the two lines have identical directory numbers except for the units digit. Thus, if ten consecutive directory numbers in a particular tens block of numbers are already assigned it will not be possible to provide the necessary directory number for the auxiliary appearance required for two-line hunting purposes. In other words, although the primarily line appearance and the auxiliary line appearance have the same published directory number as far as subscribers are concerned, the implementation of two-line hunting required that an additional directory number be removed from the reservoir of directory numbers available for assignment to other subscribers. The Masucci patent disclosed the use of a further number group frame for the purposes of providing line hunting capability in a PBX--an arrangement suitable only where a very large, concentrated number of lines are to be accorded hunting service. The provision of call waiting service for individual subscribers, however, would not justify the financial investment required to provide the auxiliary number group disclosed in the Masucci patent for large PBXs.
Accordingly, it is an object of the present invention to provide an economical method of modifying crossbar offices so that enhanced calling services, such as call waiting, may be effected without requiring the withdrawal of a directory number from the assignable pool and without the constraint of the tens block restriction which priorly characterized two-line hunting arrangements.